In geodesy a plummet serves to center geodetic instruments, for example, a theodolite, a leveling instrument or a tachymeter (especially video tachymeter), over a ground mark, for example, a survey mark or reference mark.
Such geodetic instruments are frequently mounted to a tripod plate by means of a tribrach, the tribrach supporting the geodetic instrument and allowing the instrument to be aligned horizontally. Thus, the tripod via the tribrach indirectly supports the geodetic instrument. The tripod usually consists of three feet that support the tripod plate and are each adjustable in length. The tribrach is usually configured to be movable horizontally relative to the tripod plate and to be arrestable relative to the tripod plate in a desired position.
The plummet may be integrated in the geodetic instrument or fastened to the same. In this case, a respective tribrach has a central opening so as to not obstruct the plummet. As an alternative, the plummet may be integrated in a tribrach or fastened to the same. The tripod plate of the respective tripod comprises a central opening so as to not obstruct the plummet.
Mechanical plummets (plumb-bobs) usually consist of a weight suspended from a string, with a pointed tip at the free end of the weight. The string is fastened to the geodetic instrument or tribrach such that the line defined by the string, with freely suspended weight, extends through the measuring point of the geodetic instrument if the latter is horizontally aligned. This horizontal alignment is often performed by means of two straight bubble levels disposed orthogonally relative to one another or a bull's eye level. By shifting the instrument and/or tribrach, the tip of the freely suspended weight is set in line with the desired ground mark.
Such mechanical plummets involve the disadvantage that they are susceptible to vibration or flow of air and require some time to come from a vibrating initial state to a state of rest. Moreover, they have a low accuracy of only 3 mm to 5 mm at an instrument height of 1.5 m. Therefore, optical plummets are primarily used today in geodesy.
The optical structure of the optical plummet corresponds to that of a Keplerian telescope, the eyepiece of which comprises a target mark such as a reticle. The optical plummet is positioned in the geodetic instrument or tribrach in a way that the optical axis of the beam path guided in the optical elements of the optical plummet extends at least sectionally exactly along the nadir (that is in vertical direction) if the geodetic instrument is horizontally aligned. In order not to obstruct the arrangement of the geodetic instrument and in order to provide for a convenient access from the side, the beam path of the optical plummet is often folded by 90° for this purpose. Such optical plummets have an accuracy of better than <0.5 mm at an instrument height of 1.5 m. By shifting the instrument and/or tribrach, the target mark of the optical plummet is optically brought in line with the desired ground mark.
In order to obtain the desired accuracy, the focusing of the optical plummet must be adapted to the distance from the ground mark. In non-stationary geodetic instruments this must usually be performed after every positional change. This is because the distance of the plummet from the ground mark changes.
The use of focusers to enable an adjustable focusing is very complex in optical plummets, because such a focuser must meet very high accuracy requirements to ensure that the target mark in the eyepiece is always centered in the optical axis of the beam path guided in the optical elements of the optical plummet. Therefore, only a very small play of a few micrometers is admissible for the focuser. In addition to the use of a focuser for focusing purposes, a diopter adaptation is often enabled via the eyepiece of the plummet.